286 BEHAVIOR OF THE LOWER ORGANISMS 



(3) Changes in Activity depend on Changes in Physiological States. — 

 What causes the changes in behavior described in the foregoing para- 

 graph ? Since the external conditions have not changed, the animal 

 itself must have changed. The Vorticella which contracts and folds its 

 cilia is in certain respects a different animal from the one that remains 

 extended and keeps its cilia in active motion, otherwise it would not act 

 thus differently. Its internal or physiological condition has been changed. 

 Soon its original condition is restored ; it unfolds and behaves as it did 

 at first. In the same way, the physiological condition of the Hydra 

 that stands quiet with outspread arms is different from that of the Hydra 

 which, without external cause, contracts and changes its position. The 

 behavior produced by these differences in physiological condition is the 

 same as that producible by an external stimulus. 



Other examples of changes in behavior due to changed physiologi- 

 cal states are shown in the different reactions of hungry and of 

 well-fed individuals, which we have seen in so many cases, and in the 

 different reactions of organisms as determined by their respiratory 

 processes. 



The precise nature of these internal changes of condition we of 

 course do not know. The expression "physiological states" evidently 

 includes a great many things of heterogeneous character, having merely 

 the common characteristic that they are internal modifications of the 

 living substance resulting in changed behavior. In the lower organisms 

 it is difficult to define the different classes of physiological states in an 

 objective way, though the progress of investigation will doubtless make 

 this possible. Certain fundamental differences in diverse states will be 

 pointed out in the following pages. 



(4) Reactions to External Agents depend on Physiological States. — 

 Change of activity is, of course, often produced by external agents. 

 With this point we are to deal later ; here what interests us is the fact 

 that in any given organism the reaction to a given external agent de- 

 pends on the physiological condition of the organism. This principle 

 is of such importance that we must dwell upon it. 



First we have the important fact that the reaction to a given stimulus 

 depends upon the progress of the metabolic processes. To a given 

 external condition the nature of the reaction often depends upon whether 

 it favors these metabolic processes. If material for these processes is 

 lacking, the reaction to stimuli is of such a character as to secure such 

 material. In such organisms as the ccelenterates almost the whole 

 character of the behavior, down to the details of the reactions to specific 

 stimuli, depends thus on the condition of the processes of metabolism 

 (sec Chapter XI). The behavior of organisms is similarly determined 



